The invention relates to determining the authenticity and the geographical origin of gemstones such as beryls and/or other silicates of similar crystallochemical structure.
It has been known for a long time that gemstones can be the subject of more or less elaborate treatments to improve the appearance thereof, for example to make them more brilliant, or to modify said appearance, for example to change the color of the stone. There is a very wide variety of such treatments which include heating the stone, coloring it externally, or filling in its pores or cracks with polymers. Such treatments seek to pass off a stone of poor authenticity as a stone of better authenticity. There therefore now exists a strong demand for methods that enable the authenticity of a stone to be determined and that can detect any faking with a high degree of certainty. Such methods need to be of particularly high performance given that certain treatments of the stone are difficult to detect. Naturally, such methods must not be destructive and should not run the risk of modifying the physical characteristics of the stone. There also exists a demand for methods that make it possible to determine the exact geographical origin of a gemstone in terms of its original deposit.
A known method of determining whether a stone is natural or synthetic consists in performing infrared spectroscopy on the stone and comparing the general appearance of the resulting spectrum with the appearance of the spectrum from a stone of known kind in order to determine whether the stone under investigation is of the same kind. Nevertheless, that method is not sufficiently accurate and reliable so it is to be feared that it is unsuitable for revealing certain treatments of the stone.
An object of the invention is to provide a method making it possible to determine the nature of a gemstone or to detect any treatment it has received with greater accuracy, and even to determine the geographical origin of the stone.
According to the invention, this object is achieved by a method of determining the authenticity and the geographical origin of gemstones of crystal structure, the method comprising the steps consisting in: applying an electromagnetic beam to the gemstone; determining values associated with the absorbance of the gemstone for wavelengths of the beam in an absorption direction that is predetermined relative to a characteristic axis of the crystal; calculating at least one ratio between these values; and comparing the or each ratio with predetermined corresponding ratios belonging to gemstones of predetermined authenticity and origin.
Thus, by taking account of the orientation of the crystal when studying absorption it is possible to obtain measurements of very high accuracy, thus enabling the gemstone to be characterized in very reliable manner.
In addition, performing analysis by means of one or more ratios gives rise to quantitative results. This makes it possible to characterize a gemstone by one or more magnitudes that are independent of the dimensional characteristics of the stone under study, and that are a function solely of the crystallochemical composition of the stone. This quantitative data is suitable for being compared directly with corresponding magnitudes that are known for standard stones even if they are of dimensions that are very different from those of the stone under investigation. The accuracy and the reliability of the magnitudes used for comparison make it possible to detect a very wide variety of treatments that can be applied to a stone and/or, in the absence or in the presence of such treatment, to establish with a very high degree of probability the geographical origin of the stone in terms of country or indeed of deposit.
Advantageously, the values are absorbance intensities.
Under some circumstances, it can thus suffice to measure absorbance at given wavelengths, e.g. when it is necessary merely to validate the assumed nature and origin of a stone.
Advantageously, an absorption spectrum is established corresponding to the predetermined absorption direction.
This spectrum makes it possible in particular to perform initial qualitative analysis relating to general appearance so as to preselect a certain number of likely natures and geographical origins for a stone.
Advantageously, the values are areas defined by the absorption spectrum.
Performing calculations on the basis of suitably chosen areas gives results that are more accurate than when using intensities since the results are based on the integrals of measurements and not on the measurements themselves.
Advantageously, a beam is delivered along the predetermined absorption direction.
Advantageously, a plurality of beams are applied to the gemstone along different directions, values are determined associated with absorbance in the respective directions of the beams, and the values corresponding to the predetermined absorbance direction are calculated.
This variant makes it possible to eliminate random effects that are sometimes encountered when directing the beam in the predetermined absorption direction. With this direction being known, it suffices to perform measurements for the three beams that form the three respective axes, and then to combine the results, e.g. in linear manner, so as to reconstitute the corresponding values lying on the predetermined absorption direction. This enables these values to be obtained indirectly.
Advantageously, the predetermined direction is perpendicular to the cxe2x80x94c axis of the crystal.
This direction significantly amplifies the absorption characteristic of the mineral, and this direction makes it possible to obtain experimental data that is particularly characteristic of the composition of the mineral.
Advantageously, at least one of the wavelengths is such that the associated value characterizes the presence of a specific body.
Advantageously, at least one of the wavelengths is such that the associated value characterizes the presence of a specific isotope of a specific body.
Advantageously, at least one of the wavelengths is such that the associated value characterizes the presence of a specific isotope of a specific body, said isotope being in a specific configuration relative to the crystal.
The above two variants make it possible with good accuracy to determine the geographical origin of stones or to detect any possible treatment thereof.
Advantageously, the wavelengths are situated in the infrared range.
Advantageously, the absorbance values are determined from the beam coming from the gemstone after said beam has passed through a diaphragm and/or an objective lens.
Advantageously, the gemstone is a beryl or a cordierite.
The invention also provides apparatus for determining the authenticity and the geographical origin of gemstones of crystal structure, the apparatus comprising a source of an electromagnetic beam; means for determining values associated with the absorbance of the gemstone for wavelengths of the beam in an absorbance direction that is predetermined relative to a characteristic axis of the crystal; and calculation means for calculating at least one ratio between the values.
Advantageously, the apparatus includes means for comparing the or each ratio with corresponding predetermined ratios belonging to gemstones of predetermined authenticity and origin.
Advantageously, the apparatus includes means for causing the beam that comes from the gemstone to pass through a diaphragm and/or an objective lens.